It is important to note that
the animal shown in this series of photos was taken from a trap. Animals
in the wild do not split up the back like this, rather the cephelothorax
and tail seperate on the dorsal aspect and the animal backs out through
the resulting opening. Why animals in traps shed in the manner depicted
here is a mystery. One theory is that the lack of natural substrate deprives
them of edges to use to leverage the shell off.
Another, more interesting theory is that the animal tries to postpone
molting in the trap because molting would leave it vulnerable to attack
by other inhabitants in the trap. This may sound far fetched until you
learn about the role of environment and chemical signals involved in
molting and mating (two activities that are usually linked) and why being
stuck in a trap is probably not percieved by the animal as the right
place to conduct either of these activities.
At any rate, eventually the proccess can be postponed no longer and
the shell splits, due either to changes in fluid concentrations in the
tissues, or perhaps to a growth spurt that splits the shell like a pair
of jeans that have become too tight. No one seems to know for sure.